Abstract
SUMMARY Downstream migration of Atlantic salmon (Salmo salar) smolts in river Nive South west France was studied in 1987 and 1988 to access the effectiveness of a fish bypass structure at the hydroelectric plant of Halsou. Passage of fish was determined by trapping and video recording. Daily, diurnal and hourly passages at bypass were determined. Tests with marked fish showed between 42 % and 95 % of the smolts used the surface bypass. Significantly more smolts were bypassed when the discharge was increased. Exploratory tests with halogen and mercury lights were performed. Visual observation indicated that fish was attracted to the lights but avoided the point source : results showed an increased rate of passage when the lamps lighting up the bypass were turned off.
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